First off, I hope you all are having an awesome holiday.
Something that has always bugged me about Elemental is that there isn't all that much differentiation between individuals and units. Sure, this has gotten better with the traits trees, but I still have that nagging feeling that units and heroes are pretty much interchangeable in a stack.
In Elemental and GalCiv, stack size is handled via logistics themed techs. This certainly is one approach, but I have an alternate idea...
A concept that I've seen used in at least one boardgame is the concept of a command rating/max stack size based on the ability of the command unit.
To 'port' this concept, essentially, each hero or Sov would have an associated command rating. This would be the maximum number of units that could be stacked with the Sov or hero. Such ratings would NOT stack, i.e. only the highest command rating would be used for any given stack, regardless of the number of heroes in the stack.
Armies/groups by themselves would either have no command rating, or a small one, so that say no more than 3 armies could be stacked in the absence of a Hero or Sov.
A 'beginning' hero might start with say a command rating of 4 (3 other units plus the hero). Sov's might start with a rating/max stack size of 5. Either way, this rating would be increased via either traits or level breaks (i.e. + 1 Command every 3 levels), or a traits with level requirements.
Cities could have their own 'command rating', based on city size (i.e. a Level 5 city would have, say a command rating of 8, when picking defenders, while a level 1 City might only have a rating of 4). If a hero is stationed in a city, the higher rating (city versus hero) could be used, or perhaps a 'command bonus' of +1 is added to the higher rating (so a level 5 city would have a 9 rating), to reflect the hero taking charge of the city defenders.
The 'minimum stack size' of 3 for leaderless armies could of course be higher (say 4 or 5), but you get the point of the concept.
While on the surface this might not seem like a big difference (the tech tree already does this to a point), this would encourage having heroes 'leading' stacks, hence adding to their importance. If a hero is killed in a battle, the stack would then 'break down' to appropriate stack sizes, so keeping your hero alive would be a little more important.
This would also give significance to heroes that are stationed in cities, as they would increase the number of defenders available for defense of the city.
Also, if a set of Traits is used to increase this rating, you might have some heroes that are 'command specialists', while others focus on other disciplines/have lower ratings. Summoners might have an associated 'Summoning Rating', which would dictate the maximum number of Summons they could field at any given time. The Summoning Rating would be in addition to a Summoner's command rating, but of course, the Summoner might have to choose between increasing his Summoning Rating or his Command Rating, as well as all of the other traits, when traits are chosen.
This would also tend to 'cap' the 'stacks of doom' to the number of Heroes at your disposal. You couldn't make a stack of doom just because. Also, this would encourage Heroes to 'battle each other' with their stacks, thus adding to the lore factor/epic feel of the game.
The downside, of course, is that leaderless armies would be at a disadvantage against armies with a leader at their head, due to their smaller maximum size. But I see this is a plus, in that you wouldn't want to send out army stacks out willy nilly against your foes. Instead, you'd have to plan your attacks, putting your 'generals' to use in your campaigns, and having to choose where to 'focus' your power/locate your generals/heroes. In particular, engaging in attacks/sieges against cities would pretty much require a hero to lead your stack.
I was about to try to take a crack at this via a mod for E:LH, but stack size is determined 'globally' for each Empire, not individually, so I can't think of an easy way to do this via a mod. Nonetheless, I do think that this idea has merit, hence I felt the need to share.
Well, that's my idea in a nutshell!